Tonight a sting operation is under way and it's happening online on yelp. Millions go there to get tips on where to shop and eat. As that internet traffic heats up during the holidays undercover detectives are hunting down fake reviews and shaming the businesses that posted those. How can you tell what's authentic and phony. Cecilia vega has the clues. Reporter: There is some serious cyber shadiness going on. Businesses trying to pay for their own positive online reviews to boost their bottom line. But now one well-known company is cracking down. One jewelry store was paying someone $200. Reporter: Must be some good reviews, right? This is an ongoing cat and mouse game, people are always going to try to ga system. Reporter: A very powerful system. Yelp gets more than 71 million visitors a month. But now yelp is taking on a new role -- detective, using its own undercover employees and a top secret computer filter to spot fakes. This is not acceptable behavior and frankly not just unethical but illegal and we have to let business owners know. You can't go out there and try to mislead your consumers. Reporter: Here's how it works -- this los angeles moving company posted on craigslist -- "i'm willing to pay you $50 if you can write and post a review for my business." The yelp employee posing as a writer for hire replies "i'm interested!" Then they display the evidence for all to see, a kind of scarlet letter for the internet age. The company didn't return our calls, but this san francisco scavenger hunt business let us right in. Why would you feel the need to go to that length of asking someone to write a fake review? It makes it easier for people to find you. Reporter: But the owner's plan backfired. Busted in the sting. Reporter: Busted. The yelp police. Reporter: By some estimates, as many as 30% of online reviews are complete fakes from hotels, to toys, to books. So how do you spot them? Here are the clues. Truthful reviews talk about physical space using specific details like "floor" and "small." Fake reviewers often talk mainly about themselves and their companions, words like "husband" can be red flags. As for joe garvey? Do you feel it was dishonest in any way? Yeah, I guess it was a little bit. Reporter: Was it worth doing? No, no. Reporter: Cecelia vega, abc news, san francisco.

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